The present invention relates to overvoltage and overcurrent protection apparatus for telecommunication circuitry and method of manufacturing same. In particular, the invention relates to fuses and thyristors.
Circuitry, particularly sensitive circuitry such as that found in telecommunication systems, require protection against both overcurrent and overvoltage conditions that may arise. Conditions such as short circuits may arise requiring an overcurrent protection device, such as a fuse, in order to prevent damage to circuitry.
Lightning is a common source of overvoltage in communication systems. Typically, communication systems consist of conductors in shielded cables suspended on poles or buried in the earth. The cable is made up of many conductors arranged in twisted pairs, commonly known as “Tip” and “Ring” lines for telephone systems, in particular. These cables are susceptible to transient energy from lightning and may conduct energy from the lightning to either a central office or subscriber equipment. Additionally, power sources for telecommunication systems are usually obtained from commercial power lines, which are also subject to excess energy from lightning that can, in turn, induce overvoltages in the telecommunication system being supplied by the power line.
Common approaches in the art to mitigate overcurrents and overvoltages include a combination of a fuse and a semiconductor overvoltage device such as a bi-directional thyristor, as shown in the circuit of FIG. 1. A fuse 100 is placed in series with a copper twisted pair 102 either in the Tip line 104 or in the Ring line 106. Hence, the fuse 100 protects the tip and ring wiring and also a bi-directional thyristor 110 from excessive energy in the event a continuous overvoltage is coupled to the wiring, as might occur if a power line falls across the wiring.
In order to limit overvoltage conditions, an overvoltage device such as the bi-directional thyristor 110 is connected across the twisted pair 102 in parallel with the telecommunication system 108. The thyristor 110 provides bi-directional “crow-bar” clamping of transients that may occur for either polarity. In particular, the thyristor 110 has a breakdown voltage at which a transient voltage exceeding this value will cause the thyristor 110 to begin clamping action across the lines 104 and 106. As the transient voltage attempts to rise higher, the current through the thyristor 110 will increase until a break-over voltage is reached. At this point, thyristor action is triggered and the thyristor 110 switches to its “on” or “latched” state. This is a very low impedance state that shunts or “crow-bars” the line, thereby suppressing the magnitude of the transient voltage. When the transient voltage diminishes, the thyristor 110 turns off and reverts to a high impedance “off” state.
The circuit of FIG. 1 is commonly used to protect “Tip” and “Ring” connections such as modems, telephones, facsimile machines, and line cards. While the circuit of FIG. 1 is appropriate for copper twisted pair environments, other voltage environments are also suitable for circuits sought to be protected such as alarm circuits, power supplies, remote sensors, CATV, data lines, etc.
The protection circuits used in telecommunication applications, such as that shown in FIG. 1, commonly utilize discretely packaged fuse and thyristor components connected in printed circuit wiring. The discrete component approach, however, requires that the components be properly coordinated and matched with one another in order to meet pertinent regulatory and safety agency requirements. Also, the discretely packaged components are typically sourced separately, thus adding increased cost to the final product. Furthermore, using discrete components consumes considerable physical space on a printed circuit board since two separate component packages must be placed on the printed circuit board.